1. Field of the Invention
The present invention relates to a novel layered structure manganese dioxide (MnO.sub.2) which does not transform into a spinel phase during repeated charge and discharge cycling, and to a process for producing the same by heat treating a mixture of an alkali metal compound and a manganese compound.
2. Description of the Related Art
MnO.sub.2 materials have been widely used as a cathode material for primary cells such as LeClanche cells and alkaline cells, or rechargeable cells such as lithium cells, since they are inexpensive and are not associated with any critical environmental problems. Much research concerning such MnO.sub.2 materials has been actively conducted. MnO.sub.2 materials have several different crystal structures. Among them, layered structure MnO.sub.2 materials are considered to be advantageous as cell materials since they provide a relatively favorable pathway for ionic diffusion. However, such layered structure MnO.sub.2 materials are conventionally prepared using complicated processes. Furthermore, when such layered structure MnO.sub.2 materials are used in lithium rechargeable cells, the MnO.sub.2 may transform into a spinel phase, or the layered crystal structure may be destroyed during repeated charge and discharge cycling, thereby decreasing cyclability. Phase transformation of MnO.sub.2 from a layered structure to a spinel phase may lead to a change in cell voltage and thereby cause a decrease in the cell capacity. For these reasons, the commercial use of such layered structure MnO.sub.2 materials has not been very productive. As such, the development of layered structure MnO.sub.2 materials, which have structural stability and thus do not transform into a spinel phase during charge and discharge cycling, and the process for producing the same is very important.
The former structural refinements of layered structure MnO.sub.2 materials (the crystal system and the space group) are as follows. Post et al. disclosed that layered structure MnO.sub.2 materials have a monoclinic C2/m space group (Amer. Mineral., Vol. 75, p. 477, 1990). Post et al. also reported that the crystal structure of chalcophanite (ZnMn.sub.3 O.sub.7), which is the most widely known layered structure of MnO.sub.2 has a hexagonal R3 space group (Amer. Mineral., Vol. 73, p. 1401, 1988). This report put an end to the dispute concerning the crystal structure of MnO.sub.2. Chalcophanite has been used as a model compound for analyzing the crystal structure of most layered structure MnO.sub.2 materials and was thought to have a triclinic P1 space group. Chang et al. synthesized a single crystal of Na.sub.2 Mn.sub.3 O.sub.7 having a structure similar to chalcophanite and reported that it had a triclinic P1 space group (Z. Anorg. Allg. Chem., Vol. 531, p. 177, 1985). Armstrong et al. reported that LiMnO.sub.2 produced from an ion exchange of NaMnO.sub.2 had a C2/m space group, which is the same as Post's report (Nature, Vol. 381, p. 499, 1996). Chen et al. suggested that the crystal structure of A.sub.x MnO.sub.2.nH.sub.2 O obtained from Li, Na and K permanganates by a hydrothermal method had a hexagonal R3m space group (Chem. Mater., Vol. 8, p. 1275, 1996). Croguennec et al. analyzed the crystal structure of layered structure LiMnO.sub.2 and reported that it had an orthohombic Pmmn space group (J. Mater. Chem., Vol. 5, p. 1919, 1995).
As such, layered structure MnO.sub.2 materials have been known as having various crystal systems and space groups. The differences in the crystal systems and the space groups arise from differences in synthesizing conditions, precursor, interlayer cations, etc.
In connection with the production of MnO.sub.2 for use as a cathode material in cells, layered structure MnO.sub.2 materials have been prepared via oxidation of Mn (II) or reduction of Mn (VII).
Wadsley et al. produced layered structure MnO.sub.2 materials by oxidizing manganese (II) nitrate in a strong alkaline sodium hydroxide aqueous solution with oxygen or air (J. Amer. Chem. Soc., Vol. 72, p. 1781, 1950). Parida et al. prepared a layered structure MnO.sub.2 by oxidizing manganese (II) sulfate in a weak acidic sulfuric acid aqueous solution with potassium permanganate (VII) (Electrochim. Acta, Vol. 26, p. 435, 1981). Bach et al. produced a layered structure MnO.sub.2 by a sol-gel method wherein potassium permanganate (VII) or sodium permanganate (VII) is reduced with fumaric acid (J. Solid St. Chem., Vol. 88, p. 325, 1990). Cole et al. prepared layered structure MnO.sub.2 materials by dripping concentrated hydrochloric acid into a potassium permanganate (VII) aqueous solution. Endo et al. produced a layered structure MnO.sub.2 by using a hydrothermal method from a potassium permanganate (VII) aqueous solution (Miner. Mag., Vol. 39, p. 559, 1974).
However, the above production processes have problems in that i) they require multistep reactions and thus the processes are complicated, ii) much care must be taken in controlling the temperature, concentration and pH of the reaction system, and iii) low temperature synthesis leads to a decrease in the crystallinity of MnO.sub.2 or diminishes the purity of MnO.sub.2.
In order to stabilize the layered crystal structure of MnO.sub.2 to be produced, various attempts including adding bismuth (Bi) or lead (Pb) have been conducted. Yao et al. produced layered structure MnO.sub.2 materials in accordance with Wadsley's method by adding manganese (II) nitrate together with bismuth (III) nitrate into a strong alkaline sodium hydroxide aqueous solution and then oxidizing the resulted suspension with oxygen or air (U.S. Pat. No. 4,520,005, 1985). Bach et al. prepared layered structure MnO.sub.2 materials in accordance with Cole's method by adding bismuth (III) nitrate into a potassium permanganate (VII) aqueous solution and then dripping concentrated nitric acid (Electrochim. Acta, Vol. 40, p. 785, 1995).
However, there are many problems in that the above-mentioned processes require complex multi-step reactions, and the obtained MnO.sub.2 materials have a large amount of side reaction products, and thus show a decreased crystallinity. Therefore, for the commercial use of layered structure MnO.sub.2 materials as a cathode material in rechargeable cells, there is a strong need for the development of a method for stabilizing crystal structures and preparing layered structure MnO.sub.2 materials in a simpler manner.
Many researchers have reported on lithium rechargeable cells wherein layered structure MnO.sub.2 materials have been used as cathode materials. On the other hand, most reports have pointed out that there is a problem of the transformation of MnO.sub.2 into a spinel phase.
Le Cras et al. disclosed that layered structure MnO.sub.2 materials obtained by low temperature synthesis from a sodium pennanganate solution completely transformed to a spinel phase after charge and discharge cycling (J. Power Sources, Vol. 54, p. 319, 1995). Chen et al. reported the same result as Le Cras's report (J. Electrochem. Soc., Vol. 144, p. 64, 1997). According to them, layered structure MnO.sub.2 materials obtained by decomposition of lithium permanganate (VII) at a low temperature have an oxide arrangement the same as that of a spinel phase. Such a similarity in the oxide arrangement allows the replacement of lithium with manganese, which in turn directly leads to structural instability causing the transformation of MnO.sub.2 into a spinel phase. Vitins et al. obtained a layered structure LiMnO.sub.2 by ion exchange of alpha-NaMnO.sub.2, and found a new spinel phase which is distinguished by X-ray diffraction analysis during charge and discharge cycling (J. Electrochem. Soc., Vol. 144, p. 2587, 1997).
In summary, most layered structure MnO.sub.2 materials reported up to now have the oxide arrangement similar to a cubic close packing ( . . . ABCABC . . . ) pattern, and thus may easily transform into a spinel phase having a similar oxide arrangement in the pattern of cubic close packing. For this reason, a layered structure MnO.sub.2, which does not transform into a spinel phase during repeated charge and discharge cycling when used as a cathode material in lithium rechargeable cells, and a process for producing the same are urgently needed in the lithium rechargeable cell-related industrial fields.